The present invention relates to a semiconductor device and more particularly to the structure of a peripheral circuit section of a nonvolatile semiconductor memory.
A flash memory generally includes various types of delay circuits necessary for operations, a write/erase high-voltage stabilization circuit, and a reference voltage generation circuit as well as memory cells in a chip. The flash memory therefore requires resistance elements constituting these circuits. These resistance elements generally have ohmic characteristics. The resistance elements are formed in almost the same step as that of forming a memory cell in a chip in order to improve the efficiency of a manufacturing process.
Referring to FIGS. 7A and 7B, an operation of the write/erase high-voltage stabilization circuit will be described as one example of a circuit using the above resistance elements. FIG. 7A schematically shows the write/erase high-voltage stabilization circuit, and FIG. 7B shows respective voltages generated when the circuit performs its operations. As shown in FIG. 7A, the write/erase high-voltage stabilization circuit is a feedback circuit for controlling an output voltage of a booster circuit. More specifically, resistance elements R1 and R2 are connected to an output terminal of the booster circuit including a charge pump circuit. The resistance elements R1 and R2 divide the output voltage of the booster circuit to generate a voltage Va. The voltage Va is compared with a reference voltage Vref in an operational amplifier OP1 to generate a control signal Φ1. In response to the control signal Φ1, the booster circuit is operated to control its output voltage.
If the voltage Va becomes lower than the reference voltage Vref as shown in FIG. 7B, the booster circuit in FIG. 7A operates. If the voltage Va becomes higher than the reference voltage Vref, the booster circuit stops its boost operation and a feedback operation is performed to increase the potential. The output voltage is thus held at a required voltage Vpp.
If, however, the capacitance between the resistance elements R1 and R2 and the nodes of the other elements in a semiconductor substrate is large, a delay due to the CR time constant increases. Then, the feedback operation is delayed and the output voltage greatly deviates from the required voltage Vpp. This hinders the flash memory from performing stable and high-speed operation. The smaller the capacity between the resistance elements and the nodes of the other elements, the higher the precision of the voltage stabilization circuit.
FIG. 8 is a schematic cross-sectional view of a prior art flash memory. In this flash memory, an element isolation region 22 is formed in a silicon substrate 21 and a gate oxide film 24 is formed in an element region 23 of the cell section. After that, a first gate electrode 25 is deposited. The first gate electrode 25 is used as a floating gate in the cell section and a resistance element 25a in the peripheral circuit section. In FIG. 8, reference numeral 26 indicates a first insulation film, 27 shows a second gate electrode, 28 denotes an interlayer insulation film, and 30 represents a wiring layer.
In the cell section, the first gate electrode can be formed of two layers. In the peripheral circuit section, the resistance element 25a can be formed of gate materials of the upper one of the two layers.
Since the resistance element 25a is formed on the thick element isolation region 22, the capacitance between the resistance element 25a and the nodes of the other elements in the semiconductor substrate can be decreased.
In the flash memory so constituted, the first gate electrode 25 is formed after forming the element isolation region 22. As shown in FIG. 8, therefore, the first gate electrode 25 extends to the element isolation region 22. Consequently, the element isolation region 22 of the cell section cannot be decreased in size and the elements are difficult to miniaturize further.